The present application relates generally to the field of implantable medical devices. More particularly, the present application relates thermal switches for neurostimulation devices, pacemakers, implantable defibrillators, and other implantable medical devices.
Implantable neurological stimulation devices (sometimes referred to as an implantable neurostimulator or INS) generate electrical stimulation signals that are used to influence the human nervous system or organs. Conventionally, the INS has been surgically implanted into a patient in a subcutaneous pocket in the abdomen, pectoral region, or upper buttocks area. Electrical contacts carried on the distal end of a lead are placed at the desired stimulation site (e.g., at a location in the spine or directly in the brain) and the proximal end of the lead is connected to the INS. The lead typically has one or more insulated electrical conductors (filar) that connect the electrical contacts, or electrodes, to the INS. For neurostimulation of the spine or brain, the leads may typically have 4 or 8 sets of conductors and electrodes.
It may be desirable to implant the INS at a location in the patient's head in cases where the distal end of the lead is provided at a site directly in the brain. For example, it may be desirable to implant the INS under the scalp on top of the patient's head (either on top of the surface of the skull or in a pocket formed in the skull).
Deep brain stimulation implants may be used for the treatment of a variety of diseases including epilepsy and Parkinson's Disease. In these devices, the electrodes are implanted in the brain of the patient to provide electric stimulation to affected regions of the brain.
Alternatively, the leads may be placed in the epidural region of the spine to stimulate the dorsal horn of the spinal cord for the treatment of pain, or other diseases such as angina.
These patients may also require diagnostic procedures to monitor the progression of their disease, or to aid in the diagnosis of unrelated injuries or disorders. Some increasingly common diagnostic procedures involve the use of magnetic resonance imaging (MRI). MRI systems use radio frequency radiation in the presence of a strong magnetic field to produce diagnostic images of the patient.
One problem that arises is tissue heating when the leads are exposed to RF electromagnetic fields. The leads act like antennas and absorbs energy which will be dissipated as RF currents at the electrodes. The RF currents will oscillate the water molecules near the electrodes at high frequency and cause the tissue to heat. If the amount of heating is greater than the tissue will tolerate, the tissue will be damaged. The RF heating may be especially problematic in environments such as in MRI systems where RF is used. Generally, increases in tissue temperature can lead to tissue damage. Some literature has shown the potential for heating in excess of 30° C. Helfer et al., Minimally Invasive Therapy, 2006; 15:2; 117-120.
Accordingly, there is a need to provide a stimulation lead or an implantable medical device such as an INS that will not cause unacceptable tissue heating when exposed to RF. There is further a need to provide an improved implantable medical device that utilizes a lead which disconnects the conductor(s) from the electrode(s) during and an MRI to avoid overheating. If a thermally activated switch is placed adjacent to or inside of the electrode, the RF energy in the lead will not be allowed to dissipate thru the electrodes and overheat the tissue, since the switch will open the circuit before the tissue overheats.